1. Field of the Invention
The present invention relates to a surface acoustic wave device and a communication device. More particularly, the present invention relates to an end-surface-reflection-type surface acoustic wave device using a Shear Horizontal type (SH-type) surface acoustic wave.
2. Description of the Related Art
Among surface acoustic waves which propagate along a piezoelectric substrate, as SH-type surface acoustic waves having displacement mainly in a direction that is perpendicular to the propagation direction of the surface acoustic wave, there are BGS waves (piezoelectric surface shear waves), Love waves, etc.
As an end-surface-reflection-type surface acoustic wave device using an SH-type surface acoustic wave (BGS wave), there is, for example, the device disclosed in Spring Proc. of the Acoustical Society of Japan, pp.351-352 (published in May, 1976). This surface acoustic wave device has a configuration such as that shown in, for example, FIG. 1. In FIG. 1, reference numeral 1 denotes a piezoelectric substrate, which is formed from a piezoelectric ceramic material. The top surface of the piezoelectric substrate 1 has interdigital transducers 2 and 3 disposed thereon. The interdigital transducers 2 and 3 have plural electrode fingers 2a and 2b, and 3a and 3b, which are arranged so as to interdigitate with each other (among the electrode fingers of the interdigital transducer 2e, the electrode fingers on both ends are denoted as 2b, and the electrode fingers other than those are denoted as 2a; furthermore, among the electrode fingers of the interdigital transducer 3, the electrode fingers at both ends are denoted as 3b, and the electrode fingers other than those are denoted as 3a). The arrow P indicates the polarization direction of the piezoelectric substrate 1. In this surface acoustic wave device, as a result of applying an AC voltage from the interdigital transducers 2 and 3, a BGS wave having only a displacement that is perpendicular to the surface acoustic wave propagation direction A, that is, only a transverse wave component, is excited.
Furthermore, in this surface acoustic wave device, a BGS wave is completely reflected between the free end surfaces 1a and 1b of the piezoelectric substrate 1, thereby trapping the BGS wave between the end surfaces 1a and 1b. That is, this surface acoustic wave device operates as an end-surface-reflection-type surface acoustic wave resonator. In the conventional surface acoustic wave resonator using a Rayleigh wave, reflectors need to be provided beside the interdigital transducer, whereas in the above-described surface acoustic wave device using a BGS wave, such reflectors can be omitted. Therefore, when compared to a conventional surface acoustic wave resonator using a Rayleigh wave, there is a significant advantage in that the chip size can be significantly reduced to approximately {fraction (1/10)} the size.
FIG. 2 is a sectional view of a surface acoustic wave device parallel to the surface acoustic wave propagation direction. The width of each of the electrode fingers 2a, 2b, and 3a, excluding the electrode fingers 3b at positions in contact with the end surfaces of the piezoelectric substrate 1, is λs/4 (λs is the wavelength of the surface acoustic wave). The distance between the centers of the electrode fingers 2a and 2b and the distance between the centers of the electrode fingers 3a are both equal to the wavelength λs. However, the width of the electrode fingers 3b at the ends, provided at positions in contact with the end surfaces of the piezoelectric substrate 1, is λs/8. Therefore, in this surface acoustic wave device, the distance L from the center of the electrode finger 2b positioned second from the end to the end surfaces 1a and 1b of the piezoelectric substrate 1 is L=λs/2.
The following configuration is described in the above-described reference. In the end-surface-reflection-type surface acoustic wave device, it is desirable that the end surfaces 1a and 1b of the piezoelectric substrate 1 are provided at a position at which the distance L from the second electrode finger 2b from the end is half of the wavelength λs of the surface acoustic wave (i.e., L=λs/2). If the position of the end surfaces 1a and 1b of the piezoelectric substrate 1 deviates from the position of L=λs/2, spurious vibrations are likely to occur.
Furthermore, in the paper entitled “BGS Wave Resonator Using Piezoelectric Ceramic and Applications Thereof”, Technical Report (Singaku Gihou), The Institute of Electronics, Information, and Communication Engineers (IEICE), pp.41-48, November 1996, a comparison between a surface acoustic wave device in which the end surface of the piezoelectric substrate is at a distance L=λs/2 from the second electrode finger and a surface acoustic wave device in which the end surface of the piezoelectric substrate is provided at a position deviated therefrom is described. In this paper, it is described that when the end surface of the piezoelectric substrate deviates from the most appropriate position, spurious vibrations occur.
However, in the conventional case, in the piezoelectric substrate used for an end-surface-reflection-type surface acoustic wave device, piezoelectric materials having a high relative dielectric constant, for example, LiNbO3, LiTaO3, and PZT, are used, and the most appropriate position of the end surface of the piezoelectric substrate merely means the most appropriate position at which the piezoelectric substrate having a high relative dielectric constant is used. That is, for end-surface-reflection-type surface acoustic wave devices using a piezoelectric substrate having a low relative dielectric constant, the most appropriate position of the end surface of the piezoelectric substrate has not yet been determined.